Cosmic ray feedback in the FIRE simulations: constraining cosmic ray propagation with GeV gamma-ray emission

Chan et al., available on arXiv

Abstract: We present the first results of cosmic ray (CR) feedback implemented in the Feedback In Realistic Environments (FIRE) simulations. We investigate CR feedback in non-cosmological simulations of dwarf, sub-L* starburst, and L* galaxies with different propagation models, including advection, isotropic and anisotropic diffusion, and streaming along field lines with different transport coefficients. We simulate CR diffusion and streaming simultaneously in galaxies with high resolution, using a newly-developed two moment method. We forward-model and compare to observations of gamma-ray emission from nearby and starburst galaxies. We reproduce the gamma-ray observations with constant isotropic diffusion coefficients kappa~3×10^(28-29) cm^2/s. Advection-only and streaming-only models (even allowing for super-Alfvenic streaming speeds) produce order-of-magnitude too large gamma-ray luminosities in dwarf and L* galaxies. We show that in models that match the gamma-ray observations, most CRs escape low-gas-density galaxies (e.g. dwarfs) before significant collisional losses, while starburst galaxies are CR proton calorimeters. While adiabatic losses can be significant, they occur only after CRs escape galaxies, so they are only of secondary importance for gamma-ray emissivities. Models where CRs are “trapped” in the star-forming disk have lower star formation efficiency, but these models are ruled out by gamma-ray observations. For models with constant kappa that match the gamma-ray observations, CRs form extended halos with scale heights of several kpc to several tens of kpc.